Mantle wedge control on back-arc crustal accretion

At mid-ocean ridges, plate separation leads to upward advection and pressure-release partial melting of fertile mantle material; the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away. In back-arc basins, the subducting slab is an important control on the pattern of mantle advection and melt extraction, as well as on compositional and fluid gradients. Modelling studies predict significant mantle wedge effects on back-arc spreading processes. Here we show that various spreading centres in the Lau back-arc basin exhibit enhanced, diminished or normal magma supply, which correlates with distance from the arc volcanic front but not with spreading rate. To explain this correlation we propose that depleted upper-mantle material, generated by melt extraction in the mantle wedge, is overturned and re-introduced beneath the back-arc basin by subduction-induced corner flow. The spreading centres experience enhanced melt delivery near the volcanic front, diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away. Our model explains fundamental differences in crustal accretion variables between back-arc and mid-ocean settings.     

Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency

Dimensionless numbers are important in biomechanics because their constancy can imply dynamic similarity between systems, despite possible differences in medium or scale. A dimensionless parameter that describes the tail or wing kinematics of swimming and flying animals is the Strouhal number, St = fA/U, which divides stroke frequency (f) and amplitude (A) by forward speed (U). St is known to govern a well-defined series of vortex growth and shedding regimes for airfoils undergoing pitching and heaving motions. Propulsive efficiency is high over a narrow range of St and usually peaks within the interval 0.2 < St < 0.4 (refs 3-8). Because natural selection is likely to tune animals for high propulsive efficiency, we expect it to constrain the range of St that animals use. This seems to be true for dolphins, sharks and bony fish, which swim at 0.2 < St < 0.4. Here we show that birds, bats and insects also converge on the same narrow range of St, but only when cruising. Tuning cruise kinematics to optimize St therefore seems to be a general principle of oscillatory lift-based propulsion.     

Adaptive visual metamorphosis in a deep-sea hydrothermal vent crab

Hydrothermal vents along the mid-ocean ridges host ephemeral ecosystems of diverse endemic fauna including several crustacean species, some of which undergo planktonic development as larvae up to 1,000 m above and 100 km away from the vents. Little is known about the role of vision in the life history of vent fauna. Here we report that planktonic zoea larvae of the vent crab Bythograea thermydron possess image-forming compound eyes with a visual pigment sensitive to the blue light of mesopelagic waters. As they metamorphose and begin to descend to and settle at the vents, they lose their image-forming optics and develop high-sensitivity naked-retina eyes. The spectral absorbance of the visual pigment in these eyes shifts towards longer wavelengths from larva to postlarva to adult. This progressive visual metamorphosis trades imaging for increased sensitivity, and changes spectral sensitivity from the blue wavelengths of the larval environment towards the dim, longer wavelengths produced in the deeper bathypelagic vent environment of the adults. As hydrothermal vents produce light, vision may supplement thermal and chemical senses to orient postlarval settlement at vent sites.     

An inositol tetrakisphosphate-containing phospholipid in activated neutrophils

Inositol (1,4,5)triphosphate (InsP3) and tetrakisphosphate (InsP4) have been observed in a variety of cell types and have been proposed to play roles in the receptor-mediated rise in intracellular Ca2+ (refs 2, 3). Recently, they have been shown to act synergistically in the activation of a Ca2+-dependent K+ channel in lacrimal acinar cells. InsP3 is the product of phospholipase C (PLC) action on phosphatidylinositol 4,5-bisphosphate (PtdInsP2) whereas InsP4 is believed to arise from phosphorylation of InsP3 by a cytosolic kinase. Although sought as a source for InsP4, PtdInsP3 has not been identified in any specific cell type. There were early reports of InsP4-containing phospholipids in crude extract from bovine brain, but this finding was later withdrawn. Recently, however, a membrane-bound enzyme (Type 1 PI kinase) which adds phosphate onto the 3 position of inositol phospholipids has been identified and the phosphatidylinositol-3-phosphate (PtdIns(3)P) product characterized. This suggests that several forms of phosphoinositides may exist and could be precursors for some of the variety of soluble inositol phosphate products which have been reported in recent years. Here we report the appearance of another novel phosphoinositide containing four phosphates, phosphatidylinositol trisphosphate (PtdInsP3) which we find only in activated but not in unstimulated neutrophils from human donors.